1. Field of the Invention
The present invention relates to a photoelectric conversion device, and more particularly, relates to a photoelectric conversion device having a two-dimensional photoelectric conversion region suitably used for reading (particularly, for reading with 1:1 magnification), for example, in facsimile machines, digital copiers, X-ray imaging apparatuses for nondestructive inspection, or the like.
2. Related Background Art
There are conventionally known those reading devices which use a reducing (demagnifying) optical system and a CCD type sensor as image reading devices for facsimile machines, copiers, scanners, or X-ray imaging apparatuses (so-called roentgen apparatus). With the development of photoelectric conversion semiconductor materials typified by hydrogenated amorphous silicon (hereinafter referred to as a-Si), development is remarkable of so-called contact type sensors in which a photoelectric conversion element and a signal processing portion are formed on a large-area substrate and in which an image of an information source is read by an optical system having the 1:1 (1×) magnification relative to the information source. For reading an X-ray image, X-rays are guided into a wavelength converting member such as a fluorescent member to be converted to light of wavelengths in a sensitive wavelength range of the sensor and the light is read by the sensor. Since there are optical losses in the image reading using the demagnifying optical system, the reading efficiency is intended to be increased by the contact reading with the wavelength converting member and the sensor of one magnification.
Particularly, since the a-Si is not used only as a photoelectric conversion material but is also used as a material for a thin-film field-effect transistor (hereinafter referred to as TFT), and since it thus has the advantage of capability of simultaneous formation of semiconductor layers for photoelectric conversion and for TFT, it demonstrates good matching with capacitive elements and switching elements such as TFT formed together. These elements can be formed by stacking of thin films and the stacking order of the respective thin films of these elements can be made the same. This configuration permits the thin films forming the respective elements to be formed simultaneously as common films. This simultaneous formation of the elements decreases the number of production steps and the length of routing of wires, whereby the photoelectric conversion device can be made with high SN ratio and at low cost.
Also, the capacitive elements (capacitors) can be made with good characteristics, because an insulating layer is provided as an intermediate layer between conductive layers to become electrodes. The improvement in the characteristics of the capacitive elements also realizes a highly functional photoelectric conversion device capable of outputting integral values of optical information obtained by a plurality of photoelectric conversion elements by a simple structure and also allows configurations of facsimiles and X-ray roentgen apparatuses with a large area, a high-level function, and high characteristics at low cost.
In such large-screen sensors with high characteristics, however, because of their large screen and large area, there were some cases where increase in radiation noise caused noise voltage or noise current to enter the photoelectric conversion semiconductor layers and the TFT semiconductor layers and to cause an error operation or an error signal, thereby extremely degrading the reliability of the photoelectric conversion device.
The means as shown in FIG. 1 is sometimes employed as a countermeasure against the radiation noise as discussed above.
FIG. 1 is a schematic, cross-sectional view of a photoelectric conversion device used for X-ray detection. In this photoelectric conversion device, on a photoelectric conversion device 21 having a photoelectric conversion element and TFT, there is formed a metal film 26 for formation of so-called antenna earth, as a measure for preventing the radiation noise from entering the device, by vacuum vapor deposition or the like. Numeral 23 designates a fluorescent screen for sensing X-rays, which is stuck on the metal film 26 with an adhesive 24.
It is, however, undeniable that the metal film formed by vacuum vapor deposition or the like is disadvantageous in terms of the cost. There is also room for improvement in terms of the yield.
Further, it is impossible to perfectly eliminate fine dust during fabrication of the photoelectric conversion semiconductor layers, particularly two types of dust that poses a problem; dust particles peeled off from the inner wall of a thin-film deposition apparatus during deposition of an amorphous silicon layer on the substrate; and dust particles remaining on a substrate during deposition of a metal layer on the substrate.
In addition to the circumstance that it was originally impossible to perfectly eliminate the defective condition of wire, i.e., short or open of wire, there arises the problem that the method for forming the metal film by vacuum vapor deposition or the like, and patterning it to effect wiring in order to form the antenna earth for preventing the radiation noise from entering the device would be a factor that further lowers the yield of the substrates and raises the cost in production of a large-screen photoelectric conversion device.
If the metal film is provided on the light incidence side of the photoelectric conversion portion, it will necessitate consideration on decrease in quantity of incident light due to the metal film. In order to minimize the decrease of efficiency due to this metal film, it is also conceivable to form a transparent, conductive film of a metal oxide or the like. This, however, forces more difficult formation of films and thus does not substantially solve the above problem.
As described above, the countermeasure against the radiation noise becomes more important with increase in the light-receiving area and for obtaining information with higher SN ratios.
Further, there is a need for a countermeasure which can surely prevent the radiation noise without accompanying significant increase of cost and lowering of yield.
From another aspect, the wavelength converting member such as the fluorescent screen is often rather vulnerable to an external factor such as humidity. There is, therefore, the desire for protection of the wavelength converting member in order to enhance the durability, handleability, and maintainability of the photoelectric conversion device for X-ray detection.